Directives

Topics

The directives provided by mod_headers can occur
almost anywhere within the server configuration. They are valid in the
main server config and virtual host sections, inside
<Directory>,
<Location> and
<Files> sections,
and within .htaccess files.

The directives are processed in the following order:

main server

virtual host

<Directory> sections and
.htaccess

<Files>

<Location>

Order is important. These two headers have a different
effect if reversed:

Copy all request headers that begin with "TS" to the
response headers:

Header echo ^TS

Add a header, MyHeader, to the response including a
timestamp for when the request was received and how long it
took to begin serving the request. This header can be used by
the client to intuit load on the server or in isolating
bottlenecks between the client and the server.

Header add MyHeader "%D %t"

results in this header being added to the response:

MyHeader: D=3775428 t=991424704447256

Say hello to Joe

Header add MyHeader "Hello Joe. It took %D microseconds \
for Apache to serve this request."

results in this header being added to the response:

MyHeader: Hello Joe. It took D=3775428 microseconds for Apache
to serve this request.

Conditionally send MyHeader on the response if and
only if header "MyRequestHeader" is present on the request. This
is useful for constructing headers in response to some client
stimulus. Note that this example requires the services of the
mod_setenvif module.

This directive can replace, merge or remove HTTP response
headers. The header is modified just after the content handler
and output filters are run, allowing outgoing headers to be
modified.

The optional condition can be either onsuccess
or always. It determines, which internal header table should be
operated on. onsuccess stands for 2xx
status codes and always for all status codes (including
2xx). Especially if you want to unset headers
set by certain modules, you should try out, which table is affected.

The action it performs is determined by the second
argument. This can be one of the following values:

set

The response header is set, replacing any previous header
with this name. The value may be a format string.

append

The response header is appended to any existing header of
the same name. When a new value is merged onto an existing
header it is separated from the existing header with a comma.
This is the HTTP standard way of giving a header multiple values.

add

The response header is added to the existing set of headers,
even if this header already exists. This can result in two
(or more) headers having the same name. This can lead to
unforeseen consequences, and in general "append" should be
used instead.

unset

The response header of this name is removed, if it exists.
If there are multiple headers of the same name, all will be
removed.

echo

Request headers with this name are echoed back in the
response headers. header may be a regular expression.

This argument is followed by a header name, which
can include the final colon, but it is not required. Case is
ignored for set, append, add
and unset. The header name for echo
is case sensitive and may be a regular expression.

For add, append and set a
value is specified as the third argument. If value
contains spaces, it should be surrounded by doublequotes.
value may be a character string, a string containing format
specifiers or a combination of both. The following format specifiers
are supported in value:

%t

The time the request was received in Universal Coordinated Time
since the epoch (Jan. 1, 1970) measured in microseconds. The value
is preceded by t=.

%D

The time from when the request was received to the time the
headers are sent on the wire. This is a measure of the duration
of the request. The value is preceded by D=.

When the Header directive is used with the
add, append, or set
argument, a fourth argument may be used to specify conditions
under which the action will be taken. If the environment variable specified in the
env=... argument exists (or if the environment
variable does not exist and env=!... is specified)
then the action specified by the Header directive
will take effect. Otherwise, the directive will have no effect
on the request.

The Header directives are processed just
before the response is sent to the network. These means that it is
possible to set and/or override most headers, except for those headers
added by the header filter.

This directive can replace, merge or remove HTTP request
headers. The header is modified just before the content handler
is run, allowing incoming headers to be modified. The action it
performs is determined by the first argument. This can be one
of the following values:

set

The request header is set, replacing any previous header
with this name

append

The request header is appended to any existing header of the
same name. When a new value is merged onto an existing header
it is separated from the existing header with a comma. This
is the HTTP standard way of giving a header multiple
values.

add

The request header is added to the existing set of headers,
even if this header already exists. This can result in two
(or more) headers having the same name. This can lead to
unforeseen consequences, and in general append should be
used instead.

unset

The request header of this name is removed, if it exists. If
there are multiple headers of the same name, all will be removed.

This argument is followed by a header name, which can
include the final colon, but it is not required. Case is
ignored. For add, append and
set a value is given as the third argument. If
value contains spaces, it should be surrounded by double
quotes. For unset, no value should be given.

When the RequestHeader directive is used with the
add, append, or set
argument, a fourth argument may be used to specify conditions
under which the action will be taken. If the environment variable specified in the
env=... argument exists (or if the environment
variable does not exist and env=!... is specified)
then the action specified by the RequestHeader directive
will take effect. Otherwise, the directive will have no effect
on the request.

The RequestHeader directive is processed
just before the request is run by its handler in the fixup phase.
This should allow headers generated by the browser, or by Apache
input filters to be overridden or modified.